Active load circuit

ABSTRACT

An active load circuit including a current mirror circuit used for a load to a differential amplifier, an output transistor outputting the amplified signal in response to the differential input to the differential amplifier, and a constant current source. The current mirror circuit includes a pair of load transistors and a third transistor provided to improve the circuit stability and the amplification factor. The collector of the output transistor is connected to the emitter of the third transistor to further improve the circuit stability and the amplification factor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active load circuit having improvedoperational stability, for example, when used as an active load circuitfor a differential current amplifier.

2. Description of the Prior Art

In a prior art circuit including a differential amplifier consisting ofa pair of transistors, which receive a differential input, and includinga constant current source, the circuit also includes an active loadcircuit (or bias circuit) consisting of first (A and B) and second (Cand D) pairs of transistors and a constant current source. The currentoutput from the circuit is obtained from the current flowing from theconstant current sources through the active load circuit transistors A,B, C, and D, and into and through the transistors in the differentialamplifier. In the active load circuit, transistors A and B act as acurrent mirror circuit and an active load to the amplifier, whiletransistors C and D are emitter followers, i.e., their collectors aregrounded, for the output stage. A disadvantage in the prior art circuitis that an offset current exists, and the circuit is balanced at a pointshifted by this offset current. Another disadvantage is that asufficient output current, i.e., a sufficient amplification factor,cannot be obtained, because of an insufficient change in the outputcurrent above the current differential.

In the above description, has been assumed that the amplificationfactors of transistors A, B, C and D are all equal. However, theamplification factors change, depending on the collector currents. Theamplification factor has a convex curve characteristic, i.e., anyincrease corresponds to an increase in the factor in the collectorcurrent, that is, the curve reaches a predetermined value and thendecreases. Thus, to make the amplification factors of transistors A, B,C and D all equal, the operation current for these transistors must alsobe made equal. The amplification factors of transistors A and B can bemade equal by regulating their collector currents. This collectorcurrent regulation is effected by adjusting the constant current source.The amplification factor of transistor D can also be made equal to thoseof transistors A and B by regulating D's collector current throughadjustments to the constant current source. However, the collectorcurrent of transistor C is small, because it is the sum of the basecurrents of transistors A and B, and it is difficult to adjust theamplification factor of transistor C as equal to those of transistors A,B and D, even if the characteristic of transistor C is equal to those oftransistors A, B and D.

As can be seen from the above description, the offset current in thisprior art circuit will be greater than required, and thus the circuitwill probably become more unbalanced.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an active load circuitwhich can improve operational stability and increase the value of theoutput current as compared to the input current, i.e. the gain, withoutincreasing the member of circuit elements and using a simple structure.

According to the present invention, there is provided an active loadcircuit including: first and second transistors whose emitters areconnected to a voltage source, and a third transistor whose emitter isconnected to the bases of the first and second transistors and whosebase is connected to one of the collectors of the first and secondtransistors to cause equal collector currents to flow in the first andsecond transistors. Also included is a fourth transistor whose collectoris connected to the emitter of the third transistor and whose base isconnected to the other of the collectors of the first and secondtransistors, and a constant current source connected to the emitter ofthe fourth transistor.

According to the present invention, there is also provided, an amplifiercircuit including: a first and second transistors whose emitters areconnected to a voltage source line and a third transistor whose emitteris connected to the bases of the first and second transistors and whosebase is connected to one of the collectors of the first and secondtransistors to cause the equal collector currents of the first andsecond transistors to flow. Further included is a fourth transistorwhose collector is connected to the emitter of the third transistor andwhose base is connected to the other of the collectors of the first andsecond transistors, a first constant current source connected to theemitter of said fourth transistor, and a current control device havingfirst and second current sources respectively connected to thecollectors of the first and second transistors, difference of currentsof the first and second current sources being varied in response to aninput signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will be clearlyunderstood with reference to the accompanying drawings, in which:

FIG. 1 is a prior art active load circuit;

FIG. 2 is an embodiment of the active load circuit according to thepresent invention; and

FIG. 3 is another embodiment of the active load circuit according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing the preferred embodiments of the present invention, itis necessary to describe in detail an example of a prior art circuitwith reference to FIG. 1, to ensure a clearer understanding of theadvantages of the present invention.

FIG. 1 is a diagram of a prior art active load circuit. The circuitincludes a differential amplifier DA consisting of a pair of transistorsQ₅ and Q₆ which receive a differential input IN, and a constant currentsource CS₅. The circuit also includes an active load circuit (or biascircuit) consisting of transistors Q₁ to Q₄ and a constant currentsource CS₁. The circuit in FIG. 1 provides an output current I₄ based onthe currents I₁ and I₂ which flow between the transistors Q₅ and Q₆ andthe active load circuit. The transistors Q₁ to Q₃ form a current mirrorcircuit and act as an active load to the amplifier DA. The transistor Q₄is an emitter follower for the output stage, i.e., its collector isgrounded, as is a collector of the transistor Q₃.

When each of the current amplification factors of the pnp transistors Q₁to Q₄ is β, the emitter current I_(E) of the pnp transistors Q₁ to Q₄,base currents I_(B), and collector currents I_(C) have the followingrelationships: ##EQU1## Therefore, the currents I₁ and I₂ flowing to thetransistors Q₅ and Q₆ are defined as in the following equations (1) and(2): ##EQU2## In the above equations, I_(C1) and I_(C2) are collectorcurrents of transistors Q₁ and Q₂ and are equal due to the currentmirror circuit principle, that is

    I.sub.C1 =I.sub.C2 =I.sub.C.

Accordingly, the following equation (3) is derived from the aboveequations (1) and (2). ##EQU3## As can be clearly understood fromequation (3), the output current I₄ does not equal the current I₃ of thecurrent source CS₁ even if I₁ =I₂, i.e., the differential input IN iszero. In addition, the following equation (4) is derived from theequation (3) when I₁ equals I₂ : ##EQU4## In this circuit of FIG. 1, anoffset current in equation (4) exists, in other words, the circuit isbalanced at a point shifted by the offset current. This is adisadvantage of the above-described prior art circuit.

Another disadvantage of the circuit is that sufficient output current I₄cannot be obtained, i.e., a sufficient amplification factor cannot beobtained, because the change of the output current I₄ becomes only aboveβ times the current difference (I₁ -I₂), normally β is several tens.

In the above description, it is assumed that the amplification factor βof all of the transistors Q₁ to Q₄ is equal, however, in actualpractice, the amplification factor β changes, depending on the collectorcurrent. The amplification factor β has a convex curve characteristic,i.e., any increase in the amplification factor corresponds to anincrease in the collector current, that is, the curve reaches apredetermined maximum value and then decreases. Therefore, in order tomake all of the transistors' amplification factors equal, the operatingcurrents of all of the transistors must also be made equal. Theamplification factors of transistors Q₁ and Q₂ can be made equal byregulating their collector currents. This collector current regulationis effected by adjusting the constant current source CS₅. Theamplification factor of the transistor Q₄ can also be made equal to thatof the transistors Q₁ and Q₂ by regulating the collector current for Q₄by adjusting the constant current source CS₁. On the other hand, thecollector current of the transistor Q₃ is small, because it is the sumof the base currents of the transistors Q₁ and Q₂, thus it is difficultto adjust the amplification factor β of the transistor Q₃ to be equal tothat of the transistors Q₁, Q₂ and Q₄, even if the characteristic of thetransistor Q₃ is equal to that of transistors Q₁, Q₂ and Q₄. This meansthe equation (3) cannot be realized in actual practice. As can be seenfrom the above description, the actual offset current in this prior artcircuit is greater than that shown by equation (3), and thus the circuitshown in FIG. 1 may become even more unbalanced.

FIG. 2 is a diagram of an embodiment of an active load circuit accordingto the present invention. In FIG. 2, the transistors Q₁, Q₂ and Q₃forming the current mirror circuit are the same as those shown inFIG. 1. The transistors Q₅ and Q₆ and the constant current source CS₅forming the differential amplifier circuit (DA) are also the same asthose shown in FIG. 1. The constant current source CS₁ is the same asthat in FIG. 1, and the transistor Q₄ is the same as that in FIG. 1,with the exception that its collector is connected to the emitter of thetransistor Q₃, instead of forming the emitter-follower circuit, i.e.,grounding the collector of transistor Q₄.

According to the circuit connection shown in FIG. 2, the collectorcurrent I_(C4) of the transistor Q₄ flows to the emitter of thetransistor Q₃, whereby, operating currents of the transistors Q₃ and Q₄are made equal.

As mentioned previously in the description of a prior art circuit, theamplification factors β of transistors Q₁ to Q₄ vary, depending on thecollector currents. In order to make the amplification factors oftransistors Q₁ to Q₄ equal, the operational collector currents of thetransistors must also be made equal. The collector currents oftransistors Q₁ and Q₂ are regulated by adjusting the constant currentsource CS₅. The collector current I_(C4) of transistor Q₄ is regulatedby adjusting the constant current source CS₁. The value of the emittercurrent I_(E3) of transistor Q₃ is approximately equal to the value ofthe collector current I_(C4), in accordance with the connection shown inFIG. 2. Therefore, all of the operation currents of transistors Q₁ to Q₄can be made equal, i.e., all of the amplification factors β oftransistors Q₁ to Q₄ can be made equal. Therefore, the followingequations are defined: ##EQU5## Assuming the following relationship asmentioned in the prior art circuit:

    I.sub.C1 =I.sub.C2 =I.sub.C

and substituting this into the above equations (5) to (8), the followingequation can be derived from equations (5), (7) and (8): ##EQU6## and afurther equation can be derived from equation (6): ##EQU7## Finally, thefollowing equation corresponding to equation (3) of the prior artcircuit can be obtained: ##EQU8## Equation (11) in accordance with thepresent invention proves that the output current I₄ is increased toapproximately the order of β². Considering the difference between theequations (11) and (3), the gain of the output current I₄ to thedifferential inputs in accordance with the present invention is greaterby approximately β times that of the prior art circuit shown in FIG. 1.In addition, the following equation (12) is derived from equation (11)when I₁ is equal I₂ : ##EQU9## Considering the difference between theequations (4) and (12), the offset term of the present invention issmaller by 2I_(C) than that of the prior art circuit, whereby theoperational stability of the circuit is increased.

The above embodiment with reference to FIG. 2 is an example of an activeload circuit for a differential amplifier circuit (DA). The presentinvention, however, can be applied to other embodiments.

FIG. 3 is a general circuit diagram in accordance with the presentinvention. The applifier circuit shown in FIG. 3 comprises currentcontrol means CC consisting of current sources CS₂ and CS₃, instead ofthe amplifier DA and the active load circuit mentioned above withreference to FIG. 2. The difference current between the currents CS₂ andCS₃ is controlled by the input signal IN.

The amplifier circuit shown in FIG. 3 provides a high amplificationfactor and high operational stability.

I claim:
 1. An active load circuit, comprising:first and secondtransistors each having an emitter connected to a voltage source andeach having a collector and a base; a third transistor having an emitterconnected to the bases of said first and second transistors and a baseconnected to one of the collectors of said first and second transistors,causing equal collector currents to flow in said first and secondtransistors; a fourth transistor having a collector connected to theemitter of said third transistor, a base connected to the other of thecollectors of said first and second transistors, and an emitter; and aconstant current source connected to the emitter of said fourthtransistor.
 2. An amplifier circuit, comprising:first and secondtransistors each having an emitter connected to a voltage source andeach having a collector and a base; a third transistor having an emitterconnected to the bases of said first and second transistors and a baseconnected to one of the collectors of said first and second transistors,causing equal collector currents to flow in the first and secondtransistors; a fourth transistor having a collector connected to theemitter of said third transistor, a base connected to the other of thecollectors of said first and second transistors, and an emitter; a firstconstant current source connected to the emitter of said fourthtransistor; and current control means having first and second currentsources respectively connected to the collectors of said first andsecond transistors, for varying a difference in currents produced bysaid first and second current sources in response to an input signal. 3.An amplifier circuit according to claim 2, wherein said first and secondcurrent sources comprise:a pair of transistors each having an emitter,each having a base for receiving a differential input signal, and eachhaving a collector respectively connected to said first and secondtransistors; and a second constant current source connected to theemitters of said pair of transistors.